1. Field of the Invention
The invention relates to a novel polyisocyanate from 1,4-diisocyanato-2,2,6-trimethylcyclohexane (TMCDI) which contains isocyanurate groups, to a process for the preparation, and to the use.
2. Discussion of the Background
Aliphatic and cycloaliphatic diisocyanates are widely employed in high-grade light-and weather-stable polyurethane coatings. For many applications, such as for trimerization, for example, it is advantageous for the two isocyanate groups to differ in reactivity. By virtue of this so-called selectivity the fraction of unwanted oligomers (pentamers, heptamers, nonamers, etc.) is reduced and the resulting products (polyisocyanates containing isocyanurate groups) have higher NCO contents, lower viscosities and improved technical processability.
Hexamethylene diisocyanate (HDI) exhibits no selectivity whatsoever and so the corresponding polyisocyanate containing isocyanurate groups has a high unwanted oligomer fraction. Isophorone diisocyanate (IPDI), on the other hand, is selective owing to the different stearic environment of the two isocyanate groups, but trimerization under the same conditions leads to relatively low unwanted oligomer contents of approximately 35% by weight and a trimer fraction of approximately 65% by weight.
The object was to find a trimeric isocyanate having a higher NCO content and improved, lower viscosity. This object has been achieved by the trimerization product of TMCDI, with a trimer fraction of more than 80% by weight and an NCO content of at least 19.3% by weight of the trimerization product.
It has surprisingly been found that 1,4-diisocyanato-2,2,6-trimethylcyclohexane (TMCDI), a readily available cycloaliphatic diisocyanate (J. Appl. Polym. Sci. 1994, 54(2), 207-218; EP 0 945 427 A1) is particularly suited to trimerization. For a given conversion, the unwanted oligomer content of the resulting polyisocyanates containing isocyanurate groups is distinctly lower than in the case of polyisocyanates based on IPDI and containing isocyanurate groups, and the NCO content is distinctly higher.
The invention provides polyisocyanates containing isocyanurate groups from 1,4-diisocyanato-2,2,6-trimethylcyclohexane, having a trimer fraction of more than 80% by weight preferably more than 90% by weight and an NCO content of at least 19.3%, preferably at least 19.6% by weight.
The invention further provides a process for preparing polyisocyanates containing isocyanurate groups from 1,4-diisocyanato-2,2,6-trimethylcyclohexane, having a trimer fraction of more than 80% by weight, preferably more than 90% by weight, and an NCO content of at least 19.3%, preferably at least 19.6% by weight, by reacting TMCDI in the presence of at least one trimerization catalyst at from 70 to 150xc2x0 C.
1,4-Diisocyanato-2,2,6-trimethylcyclohexane, referred to below simply as TMCDI, may be converted into a polyisocyanate containing isocyanurate groups by trimerization in the presence of appropriate catalysts. For this purpose, a catalyst is added to TMCDI at room temperature and the system is heated at 130xc2x0 C. After twenty minutes, depending on the amount of catalyst, the reaction has progressed to a conversion of approximately 30-60% and is terminated either thermally or by adding an acid. Excess diisocyanate is distilled off at 150xc2x0 C. and 0.1 mbar in a short-path evaporator. The resulting polyisocyanates containing isocyanurate groups have an unwanted oligomer fraction of 8-20% by weight (trimer fraction 92-80% by weight) and an NCO content of 19.3-20.0% by weight.
Non-limiting examples of suitable trimerization catalysts are quaternary ammonium carboxylates, alkali metal and alkaline earth metal salts of carboxylic acids, trialkylphosphines or quaternary ammonium carbonates or amino silane compounds. Trioctylmethylammonium 2-ethylhexanoate, for example, is particularly suitable, and may be prepared from the corresponding chloride by the method described in more detail in U.S. Pat. No. 5,691,440.
TMCDI-based polyisocyanates containing isocyanurate groups, possibly in blocked form, represent valuable starting materials for preparing polyurethane plastics by the isocyanate polyaddition process, as base materials for coatings, and especially for preparing one- or two-component polyurethane coating materials.
Suitable blocking agents are known within isocyanate chemistry, examples being ethyl acetoacetate, diisopropylamine, methyl ethyl ketoxime, diethyl malonate, E-caprolactam, 1,2,4-triazole, and 3,5-dimethylpyrazole.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. The examples which follow are intended to illustrate, but not restrict, the invention:
Preparation of a polyisocyanate containing isocyanurate groups from 1,4-diisocyanato-2,2,6-trimethylcyclohexane.
292 g of 1,4-diisocyanato-2,2,6-trimethylcyclohexane were introduced into a reaction vessel, 1.46 g of trioctylmethylammonium 2-ethylhexanoate were added, and the system was heated at 130xc2x0 C. under insert gas and with stirring. After 20 minutes, the temperature was raised briefly to 150xc2x0 C. and the system was then cooled (NCO content: 31.3%, 45% conversion). Excess TMCDI was separated off on a short-path evaporator at 150xc2x0 C. and 0.1 mbar. The demonomerized product had an NCO content of 19.8% and an unwanted oligomer fraction of 33% (trimer fraction 92%) (gel permeation chromatography (GPC), super-fluid chromatography (SFC)).